1. Field of the Invention
The invention relates to a receiver for receiving high-frequency signals, comprising a receiving section which includes a first frequency loop with a frequency conversion device for frequency conversion of a high-frequency modulated signal, to be applied to the receiving section, into a modulated signal of lower frequency, a detection device for detecting an information signal present in the modulated signal of lower frequency, and a tuning oscillator which is coupled to the frequency conversion device and whose frequency can be adjusted by means of a first tuning control signal which can be supplied by a first tuning control signal generator coupled to the detection device. A receiver of this kind may be a radio receiver, a television receiver, a video recorder, or a cordless or mobile telephone.
2. Description of the Related Art
A receiver of this kind in the form of a radio receiver is known as an integrated "AM/FM Radio Receiver Circuit", TEA5592, described in the Philips Data Handbook "Radio, audio and associated systems Bipolar, MOS", Book IC01, 1991, pp. 1047-1060. An application of this IC is shown on page 1057 of said handbook, i.e. as an AM/FM radio receiver from aerial signal to low-frequency or audio signal. Tuning of the AM/FM radio receiver is performed manually by means of two ganged capacitors. For FM there is provided a tuning control signal generator FM-AFC, as shown on pages 1049 and 1057, which is coupled to the FM discriminator which serves as demodulator. The FM-AFC supplies a tuning control signal for the FM tuning oscillator which is coupled to a frequency conversion device, in this case being an FM mixer whereto a high-frequency modulated signal can also be applied. The tuning control signal serves to adjust the oscillator frequency, within a given range, so that fine tuning is achieved after manual tuning which could otherwise lead to imperfect tuning to a radio station. Therefore, there is provided a first frequency loop which serves for fine tuning to a radio station to be listened to. The AFC ("Automatic Frequency Control"), ensures that despite drift phenomena due to temperature effects, voltage variations and the like, the frequency of the tuning oscillator retains a value enabling optimum operation of further signal processing means, such as an intermediate frequency stage in a superheterodyne receiver. The AFC utilizes the so-called S-curve FM discriminator characteristic. Any variation from the central frequency of the FM discriminator gives rise to DC voltage variations at the output of the discriminator, with a polarity which is determined by the direction of the frequency deviation. This DC component is separated from the desired discriminator output signal by means of a low-pass filter, after which it is applied to the local oscillator so that the frequency of the local oscillator is varied in a direction tending to correct the error in the mean intermediate frequency. Tuning errors due to inter alia drift phenomena can thus be eliminated to a high degree and accurate tuning is achieved, any selective intermediate frequency amplifiers present in the radio then operating optimally in respect of signal processing. It is a drawback of the known AFC that hysteresis occurs in the tuning, i.e. notably in the case of a busy frequency band it is difficult to tune to a desired station because, depending on the search direction in the frequency band, the AFC often acts on a station in the immediate vicinity of the desired station. A further drawback consists in that in the case of fading, i.e. a temporary strong reduction of the signal received, notably of importance for portable radio receivers such as portable broadcast receivers and radio receivers in cordless telephones or car telephones, so-called "pulling" to a neighbouring station or radio channel can occur under the influence of the AFC.
From U.S. Pat. No. 4,344,187 there is also known an electronically tunable radio receiver in which the tuning function is provided by a phase locked loop synthesizer which cooperates with an AFC in an intricate manner, thus precluding suitable compromise between the respective loop parameters. The AFC is switched on only when a signal of adequate strength is received; this is detected via a muting control function. Using the PLL tuning, radio stations in a frequency band are step-wise searched, the frequency steps being adjusted by means of a programmable divider in the PLL. Even though not shown, control signals for tuning in such electronically tunable radio receivers are usually supplied by a microprocessor. The operation of such PLL tuning is as follows. The phase of the signal of the voltage-controlled tuning oscillator, divided by a dividend, is compared with the phase of a reference clock signal which is generated by a crystal oscillator. The error signal of the phase comparator, being a series of narrow pulses, is integrated in a loop filter which supplies a control voltage for the local oscillator. A different tuning frequency can be chosen by adjustment of a different dividend. A drawback which is of importance notably in portable receivers consists in that the programmable divider has a comparatively high power consumption, inter alia because of the feedbacks in the divider which are required for implementing arbitrary dividends. Moreover, the PLL should be carefully designed in respect of noise behaviour in order to ensure that the local oscillator signal-to-noise ratio is not restricted by the tuning system. Furthermore, due to the skipping of narrow pulses in the case of small phase errors, the charge pump in the loop has a dead zone, so that the output signal of the local oscillator exhibits drift phenomena. The PLL residual errors due to this dead zone have a comparatively large bandwidth, so that the time constant of the filter should be comparatively large so as to filter out resultant interference signals, thus leading to a slower tuning system. Therefore, a compromise must be found between interference and speed. Integration of the tuning system with the receiving section in one integrated circuit also poses major problems because of the interference between the tuning system and a receiving section. Because of the step-wise tuning, the tuning quality obtained is inferior to that obtained in radio receivers with manual tuning and AFC, like said TEA5592. Furthermore, PLL tuning is comparatively slow; this is a drawback notably for application in RDS car radios and in cordless telephones or mobile radios. Moreover, PLL implementations in integrated form still require a comparatively large number of external components.